Scientists Watch a Fish Think

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For the first time, scientists have imaged the brain activity of
a fish watching its prey.

Observing
neural signals in real time offers an important glimpse into
how brains perceive the outside world. In the new study,
researchers developed a way to follow these signals in the brain
of a zebrafish larva, using a sensitive fluorescent marker.

"It's a breakthrough," molecular and cell biologist Florian
Engert of Harvard University, who was not involved in the study,
told LiveScience. "No one else can look at neuronal activity with
fluorescence microscopy in a freely swimming zebrafish larva"
with such good resolution.

See-through heads

Zebrafish are widely used to study genetics and development in
vertebrates. Their larvae are ideal for
neuroimaging because they have translucent heads, and
scientists can literally peer into their brains.

To see what was actually going on in those fish noggins,
researchers developed a genetically engineered protein, called
GCaMP7a, that lights up under a fluorescent microscope when
neurons, or brain cells, fire. Transgenic zebrafish were bred to
express this protein in a brain region called the optic tectum,
which controls the movement of the eye when the animal sees
something move in its environment.

In one experiment, the scientists imaged the brain of a
transgenic fish larva as it watched a dot on a screen blinking on
and off or moving back and forth. Under the microscope, signals
flashed through the fish's brain, mirroring the movement of the
dot. [See video of the fish's brain.]

Next, a live paramecium — zebrafish prey — was placed in sight of
an immobilized fish. Again, neural signals could be seen zipping
around the fish's brain, tracking the paramecium's movement. No
signals were detected when the paramecium was motionless,
however.

Lastly, a paramecium was placed in a dish with a zebrafish larva
that was allowed to swim freely, hunting its prey. The
researchers mapped the fish's brain activity as it zeroed in on
the paramecium and swam toward it.

Understanding brain behavior

The new approach will improve scientists' understanding of
brain
circuits involved in predatory behavior, the researchers
report online today (Jan. 31) in the journal Current Biology. The
system could be used to image other brain areas, too, allowing
scientists to
observe neurons involved in behavior and locomotion.

Previously, scientists had been able to image single-cell brain
activity in zebrafish, but this study was the first to do it in a
freely swimming fish perceiving a natural object. "The technology
for studying zebrafish is moving fast," said neuroscientist
Joseph Fetcho in an email to LiveScience. Fetcho did some of the
earlier imaging work but was not involved in the new study.

The closer one can get to revealing the patterns of neuronal
activity in a freely behaving animal, the more likely the
patterns will represent those that drive natural behavior, Fetcho
said.